1. Field of the Invention
The present invention relates generally to an exposure apparatus employed for manufacturing a semiconductor device and a liquid crystal display substrate or the like and, more particularly, to an exposure apparatus for performing, a so-called picture synthesization in which a pattern having a large areal size is formed by overlapping some portions of unit-area patterns with each other on a photosensitive substrate.
2. Related Background Art
This type of exposure apparatus has hitherto involved the use of a picture synthesizing method of eventually synthesizing a desired pattern by partitioning an exposure area into a plurality of unit areas on a photosensitive substrate and repeating an exposure in accordance with each unit area in order to cope with a scale-up (increase in areal size of the exposure area) of the photosensitive substrate as an object for exposure. When effecting this picture synthesization, a positional deviation of the pattern is caused derived from a pattern depicting error of a reticle for a pattern projection, an aberration in a projection optical system and a positioning error of a stage for positioning the photosensitive substrate. For this reason, it follows that pattern breaks appear in positions of boundaries between the respective exposure areas, which should be originally connected without any gap. For preventing this, very slight quantities of boundaries between the respective exposure areas are overlapped with each other, thus effecting the exposure. When the exposure areas are overlapped, however, an exposure quantity of this portion is doubled, resulting in variation in terms of line width of the pattern at the connected portion between the respective exposure area depending on a characteristic of a photosensitive agent. Further, when the picture synthesization takes place, a stepped portion is produced at the pattern joint lines due to a positional deviation (deviation in the direction orthogonal to the direction in which the pattern extends) between the exposure areas adjacent to each other. It may happen that the characteristic of the device is to be spoiled. Further, if a different exposure apparatus is burdened per layer (per process) with a process of overlapping the picture-synthesized single-layered patterns with each other in multi layers, an overlap error of the exposure area of each layer discontinuously changes at the pattern joint lines, depending on the aberration of the lens and a difference in terms of a positioning accuracy of each exposure apparatus. As a result, especially in an active matrix liquid crystal device, a contrast discontinuously varies at the pattern joint lines, resulting in a decline of a device quality.
A method of obviating the problems inherent in the picture synthesization described above entails a technique disclosed in Japanese Patent Publication No. 63-49218. According to this technique, light reducing or light attenuating portions for reducing a quantity of the transmitted light are provided in positions, corresponding to the pattern joint lines, of a reticle or a filter superposed on the reticle. Then, pattern images passing through the light reducing portions are overlapped with each other on the photosensitive substrate, and, thus, the exposure is effected so that an exposure quantity of the pattern overlapped portion is substantially coincident with the exposure quantity of other portions.
The following problems, however, arise, in the technique stated in the above Publication.
First, when the reticle itself is provided with a light reducing or light attenuating characteristic, the number of steps of manufacturing the reticle increases, and a burden on the reticle manufacturing steps augments as in the case of increasing the possibility in which a pattern defect is to be produced during the manufacturing process. On the other hand, when using the filter to be overlapped with the reticle, there increases such a possibility that the reticle is damaged and contaminated by attaching and detaching the filter. Thus, a problem is produced in terms of a maintenance and a management of the reticle. Further, a dedicated filter is required to be prepared per reticle, and labors needed for manufacturing, maintaining and managing the filter are not ignorable.